MCB tripping.

[user 38059]

Hi everyone, Im a qulified sparky with over 40 years experience. Ive got a small problem that is bothering me. I wired in a 32 amp DP isolation swtich with neon indicator on to a ring main circuit. With the intention of being able to switch off the entire ring main locally to the room which the ring main serviced, this is my own house. I did it by sending the 32 amp feed a few metres from the CU via a 32amp mcb using 4mm twin & cpu as the supply cable to the 32amp DP switch. Another 4mm cable leaving the switch on the load side which connected to both end legs of the ring. This gives me the ability to swich off all the equipment plugged into the ring main with one flick of a switch. The load on the circuit isnt high no more than 6-amps max. However often when I turn on the DP switch the mcb trips out. If I then re set the mcb whike the DP switch is still in the on position the mcb holds in and remains on with out further issues. The circuit is free from any fault conditions and so I conclude it has to be some anomoly with the equipment thats plugged in. Rationally one would conclude that the start up load was too high for the breaker but that is not the case here, most of the equipment is in standby mode IE they are actually switched off on the units themselves. All ideas or thoughts are welcome.

 

[user 38059]

Hi everyone, Im a qulified sparky with over 40 years experience. Ive got a small problem that is bothering me. I wired in a 32 amp DP isolation swtich with neon indicator on to a ring main circuit. With the intention of being able to switch off the entire ring main locally to the room which the ring main serviced, this is my own house. I did it by sending the 32 amp feed a few metres from the CU via a 32amp mcb using 4mm twin & cpu as the supply cable to the 32amp DP switch. Another 4mm cable leaving the switch on the load side which connected to both end legs of the ring. This gives me the ability to swich off all the equipment plugged into the ring main with one flick of a switch. The load on the circuit isnt high no more than 6-amps max. However often when I turn on the DP switch the mcb trips out. If I then re set the mcb whike the DP switch is still in the on position the mcb holds in and remains on with out further issues. The circuit is free from any fault conditions and so I conclude it has to be some anomoly with the equipment thats plugged in. Rationally one would conclude that the start up load was too high for the breaker but that is not the case here, most of the equipment is in standby mode IE they are actually switched off on the units themselves. All ideas or thoughts are welcome.

Hi everyone, Im a qulified sparky with over 40 years experience. Ive got a small problem that is bothering me. I wired in a 32 amp DP isolation swtich with neon indicator on to a ring main circuit. With the intention of being able to switch off the entire ring main locally to the room which the ring main serviced, this is my own house. I did it by sending the 32 amp feed a few metres from the CU via a 32amp mcb using 4mm twin & cpu as the supply cable to the 32amp DP switch. Another 4mm cable leaving the switch on the load side which connected to both end legs of the ring. This gives me the ability to swich off all the equipment plugged into the ring main with one flick of a switch. The load on the circuit isnt high no more than 6-amps max. However often when I turn on the DP switch the mcb trips out. If I then re set the mcb whike the DP switch is still in the on position the mcb holds in and remains on with out further issues. The circuit is free from any fault conditions and so I conclude it has to be some anomoly with the equipment thats plugged in. Rationally one would conclude that the start up load was too high for the breaker but that is not the case here, most of the equipment is in standby mode IE they are actually switched off on the units themselves. All ideas or thoughts are welcome.

Sorry for the typos, should say CPC not CPU.

 

[Fleeting]

I suspect the poles at the switch aren't making simultaneously creating an imbalance. Is this with the equipment plugged in?

 

[Geoff1946]

It is just an MCB ? Not an RCBO ? I can visualise how it could happen with RCBO, but no idea otherwise.
What happens if you unplug everything from the ring ? I'm just thinking some capacitive power supplies have a serious inrush surge, though tripping a 32 amp breaker is surprising.

 

[Fleeting]

Didn't spot the word mcb so that makes a difference.

 

[Murdoch]

Didn't spot the word mcb so that makes a difference.

Huge difference

Dare I asked what testing you have done ?

Ho hum

 

[Fleeting]

Huge difference

Dare I asked what testing you have done ?

Ho hum

I haven't done any testing.

 

[Andy™]

so what testing has been done? did ecerything work before the switch was added?

 

[Murdoch]

I haven't done any testing.

ha ha

that was aimed at the OP

 

[Phoenix]

Whats the load? if its lots of electronic equipment with switch mode power supplies, then equipment being in standby will not make a difference to the inrush, the capacitors on the inrush stage still have to charge up, I expect you'd get the same if it had been off at the mcb for more than a few minutes and you turned it on using the mcb (with the dp switch already in the on postion) rather than the DP isolator.

If you can meet Zs for it, you could change to a C type MCB, however you'll probably start to find that the 32A DP isolators are not great for loads such as this, generally the headline rating is at AC1, I've seen in a school that had this sort of set up for banks of computers in an IT base, that most of the 32A switches had the contacts welded closed and couldn't be switched!

 

[Apache]

The DP switch must be faulty. What make is it? Did you buy it specifically, or is it old stock?

I'd be changing that/returning that.

Is the MCB new? What make?

 

[roys]

Whats the load? if its lots of electronic equipment with switch mode power supplies, then equipment being in standby will not make a difference to the inrush, the capacitors on the inrush stage still have to charge up, I expect you'd get the same if it had been off at the mcb for more than a few minutes and you turned it on using the mcb (with the dp switch already in the on postion) rather than the DP isolator.

If you can meet Zs for it, you could change to a C type MCB, however you'll probably start to find that the 32A DP isolators are not great for loads such as this, generally the headline rating is at AC1, I've seen in a school that had this sort of set up for banks of computers in an IT base, that most of the 32A switches had the contacts welded closed and couldn't be switched!

How did you get round that Phoenix, fit an AC3 contactor per chance.
OP are you powering up something like a 240/110V large site transformer?

 

[Phoenix]

I think in the end, it was decided that the client didn't actually require the switches, they had not been able to opperate for so long, that they'd just been left on for however long it had been, so I beleive they just got linked out and blank plated in the end. If they had wanted to keep them, the contactor would have likely been the best option. Or a lot of rotary isolators also have a rating at AC-3, although its often quite a bit less then the headline rating, so could have gone with an oversized rotary isolator mounted above the dado, and there might have even been some advantage using a 4-pole one and wiring two sets of poles in series so as to effectivly double the contact opening speed / halve the arc voltage (depending on which way you look at it), probably not strictly necessary if you get one with a suitable AC-3 rating but would not hurt, and I seem to remember back in the early days of solar PV, some DC isolators were AC isolators with poles in series tested and re-certified for DC when connected in that configuration

 

[Geoff1946]

If the original problem of the MCB tripping WAS caused by multiple equipment's inrush surges, then it's worth pointing out that unnecessary switching of such equipment is not advisable anyway. Equipment with standby mode is designed to be continuously powered. IMO the stresses to components of repeatedly switching on is likely to shorten it's life.

 

[user 38059]

Thanks for all your comments although none seem very helpfull, all equipment is individually swtiched off with the exception of 1 small negative ion generator and 3 small dc transformers no more than 0.5amp each. If it was in rush current (which it isnt) the mcb would trip when I switched it back on, which it doesnt. The circuit itself is fault free Ive checked it ( I have 2391). I think perhaps the best answer is to change the MCB and the 32smp switch, which does seem a bit clunky. Changing for a motor rated breaker seems a bit pointless as there is virtually no start up load.

 

[Murdoch]

I’m a bit lost as to why you did this?

I know you say it’s a MCB which sounds very odd, more like the behaviour of a RCBO

 

[user 38059]

Its an MCB not an RCBO also the reason why I did it has no relevence to why there is a problem. Rest assured I had a good reason.

 

[Murdoch]

Its an MCB not an RCBO also the reason why I did it has no relevence to why there is a problem. Rest assured I had a good reason.

Not sure I agree with this and as you are being less than forthcoming with information I think I’ll get my coat

 

[user 38059]

Not sure I agree with this and as you are being less than forthcoming with information I think I’ll get my coat

You dont agree that the device that is tripping is an MCB

 

[John Burns]

I assume the final ring was off the 32A mcb in the CU and all was well. Then the 4mm radial cable was installed to the 32A switch. I assume 4mm leaves the 32A switch to a junction box, then the final ring off the junction box.

The additions are: the switch, short runs of 4mm cable and a junction box (assumed). The only significant addition is the switch.

A 4mm cable can typically carry a current of up to 32 amps under normal conditions. This can vary depending on the cable's insulation material, ambient temperature, and installation method.

I would have:
1) Fitted a 40A switch, 32A is cutting it too fine.
2) Used 6mm cable, as 4mm may be cutting it too fine.
3) Taken the final ring from the output terminal of the 40A switch rather than use a junction box. Or rather than use a screwed terminal junction box use superior screwless Wago connectors.
4) Use ferrules on all terminations.